This invention is directed to thermoplastic compositions having improved mechanical properties such as notched Izod impact and comprising a polycarbonate polymer, a polyester polymer, and optionally a polylactic acid polymer.
Thermoplastics having good mechanical properties such as notched Izod impact strength are useful in the manufacture of articles and components for a wide range of applications, from automobile components, to decorative articles, to housings for electronic appliances, such as computers and cell phones. Due to environmental considerations and limitations in petrochemical resources and supplies, it is also desired that such thermoplastic compositions be based, at least in part, on potentially renewable raw materials such as natural polymers or “biosourced” polymers made, at least in part, from such renewable raw materials as succinic or lactic acids.
Publication JP2006-111858 discloses complex resin compositions based on polylactic acid said to have favorable impact strength, color, and flame retardance properties. The complex resin compositions require the use of a complicated mixture of numerous special components including a polylactic acid resin, a cellulose ester, an aromatic polycarbonate resin, a compatibilizer, a flame retardant, a fluorine-based compound, and an epoxy compound. In addition to the complexity concerning the number of required components in this composition, the document also specifically warns that great care must be taken in using very specific amounts of some of the components, and in many cases even their specific characteristics; otherwise problems such as poor processability, miscibility, thermal stability, and mechanical stability may result.
Publication JP2002-371172 discloses a polylactic acid resin composition said to have favorable stiffness, toughness, heat resistance, appearance and moldability. The disclosed resin composition comprises a polylactic acid and a polycarbonate, especially an aliphatic polyestercarbonate as a raw material, and it is disclosed that this composition is obtained by subjecting the polymers to a crosslinking reaction using a peroxide to compatibilize the polymers. In particular, the resin composition is obtained in a process of melting and mixing the polylactic acid, the polycarbonate and a radical reaction initiator under a nitrogen atmosphere. Specifically this document warns that this crosslinking process is complex, and one must take care in feeding and mixing these components so that undesired decomposition reactions or gel formation problems do not occur. In addition, it is disclosed that the selection of an appropriate initiator and the means of adding it are critical so that the initiator does not thermally decompose prior to bringing about the desired crosslinking.
U.S. Pat. No. 5,847,011 discloses the preparation of degradable copolymers comprising an aromatic polycarbonate block and an aliphatic polyester block by subjecting an aromatic polycarbonate and an aliphatic polyester to a reaction in the presence of a catalyst in a molten state or dissolved in an organic solvent. The degradable copolymers were stated to be distinctly different from the composition obtained by blending an aromatic polycarbonate and an aliphatic polyester. For example, they have both a single glass transition temperature and a single weight average molecular weight. In addition, the impact strength properties of such degradable copolymers are generally expected to be worse than those of a blend of the aromatic polycarbonate and an aliphatic polyester. Although the degradable copolymers have some desirable properties such as transparency and improved rates of biodegradability, nonetheless their preparation from an aromatic polycarbonate and an aliphatic polyester typically requires a reaction step of many hours at elevated temperatures and at reduced pressures under reflux conditions in a special reaction system, followed by a quenching reaction step with an acid, and finally a drying step to remove the various solvents and reaction byproducts.
There accordingly remains a need in the art for thermoplastic compositions based on raw materials obtainable from renewable and/or plant sources such as polyesters of succinic acid. These thermoplastic compositions should have improved mechanical properties and should not require complex mixtures of components. Another desirable feature is ease of manufacture, such as not requiring extensive reaction and purification steps. It will be shown that the mechanical properties of the thermoplastic compositions of this invention are desirably comparable to those of other thermoplastic polyester/polycarbonate blend compositions. Furthermore it will be shown that the compositions of this invention can be readily made through the use of simple blending processes known in the art and that even relatively simple mixtures of the specified components have desirable properties. In addition, polylactic acid, which is a biosourced polymer, may optionally be incorporated in the thermoplastic compositions of this invention at low levels.